With regard to electrophotographic photoreceptors, organic photoreceptors, which exhibit advantages in environmental protection as well as ease of production, have been mainly employed instead of inorganic photoreceptors. Currently, organic photoreceptors, utilizing various materials, have been developed.
In recent years, separate function type photoreceptors have played the main role in which charge generation and charge transport function employ different materials. Of these, laminated-layer type organic photoreceptors (hereinafter occasionally referred to simply as photoreceptors) are widely employed in which a charge generating layer and a charge transport layer are laminated to each other.
Further, when attention is paid to electrophotographic processes, techniques are directed from analogue image formation, employing halogen lamps as a light source, to digital system image formation, employing LED as well as lasers as a light source, due to ease of image processing as well as ease of application in composite machines.
The feature of such digitals copiers is a function which produces copies, utilizing electronic data. Due to that, it is possible to employ digital copiers as printers. Listed as image forming methods in such digital copiers are the following methods. Original document images of several pages are read employing imaging elements such as CCD; the resulting image data (hereinafter occasionally referred to as an electronic image) are stored in memory; image data are read from said memory; and images are formed onto recording paper (referring to final image recording paper and image supports such as transparent sheets).
In addition, another feature of digital copiers is that it is possible to use a so-called electronic RHD (Recirculating Document Handler) due to electronic capability of printing on both sides, as disclosed in Japanese Patent Open to Public Inspection No. 2001-147547. Since the electronic RDH makes it possible to electronically print both sides, it is unnecessary to store recording paper sheets which are printed on one side, being different from the double sided printing system employed in conventional copiers, whereby it is possible to continuously print one side and then the other side of recording paper sheets. Namely, a latent image is digitally formed and a toner image is then formed on a photoreceptor. The toner image is transferred onto one side of a recording paper sheet and fixed. Immediately after passing through the fixing process, the resulting recording paper is transported to the transfer and fixing process for the other side. As a result, when thermal fixing is employed, the recording paper sheet, heated at relatively high temperatures, is transported to the transfer and fixing process for printing on the other side, immediately after passing the fixing process. In such a case, the temperature in the interior of the apparatus increases due to heat retained by the recording paper. Subsequently the temperature of the photoreceptor increases. As a result, problems in terms of images tend to surface, which do not occur at room temperatures.
In image formation utilizing the digital system, reversal development is generally performed in which an exposed portion is subjected to a toner image. In reversal development, black spots which are peculiar to reversal development tend to be visualized when the temperature of the photoreceptor increases. The organic photoreceptor generally comprises an electrically conductive support having thereon a laminated layer structure comprised of an interlayer, a charge generating layer, and a charge transport layer. In order to overcome the black spot problem, techniques have been developed which minimize the injection of charge carriers from the electrically conductive layer to the interlayer. For example, an electrophotographic photoreceptor is known having a structure in which an interlayer is provided between the electrically conductive layer and the photosensitive layer, and in that interlayer, titanium oxide particles are dispersed in the resins. Further, also known is an interlayer in which surface-treated titanium oxide is incorporated. For example, organic photoreceptors are proposed which comprise an interlayer in which titanium oxide, which is subjected to a surface treatment employing iron oxide and tungsten oxide as disclosed in Japanese Patent Open to Public Inspection No. 4-303846, titanium oxide which is subjected to a surface treatment employing amino group containing coupling agents as disclosed in Japanese Patent Open to Public Inspection No. 9-96916, titanium oxide which is subjected to a surface treatment employing organic silicon compounds as disclosed in Japanese Patent Open to Public Inspection No. 9-258469, titanium oxide which is subjected to a surface treatment employing methylhydrogenpolysiloxane as disclosed in Japanese Patent Open to Public Inspection No. 8-328283, and dendritic titanium oxide which is subjected to a surface treatment employing metal oxides or organic compounds as disclosed in Japanese Patent Open to Public Inspection No. 11-344826, are incorporated.
However, at an ambience of high temperature and high humidity, generation of black spots is not sufficiently minimized. Alternatively, an increase in residual potential as well as an increase in potential in the exposed portion occurs during repeated use. As a result, problems occur in which it is difficult to achieved sufficient image density.
At an ambience of high temperature and high humidify, when black spots are minimized by increasing insulation of the interlayer, so that the transfer of free carriers from the electrically conductive support to the photosensitive layer is minimized, problems are noticed in which, contrary to the black spots, other image defects, called undeveloped spots, tend to occur. The undeveloped spots, as described herein, refer to image defects such as undeveloped spots or streaks in halftones or black solid images formed by reversal development. It is assumed that the phenomena occur in such a manner that during latent image formation on an organic photoreceptor, minute spots, in which charges are not eliminated, are formed in the area which is subjected to image exposure, and are the reverse phenomena with respect to the aforesaid black spots. In the image forming apparatus utilizing such a photoreceptor, image problems occur such as black spots on white backgrounds and white undeveloped spots on black backgrounds or halftones, which are not compatible with each other. Accordingly, it has been demanded to develop organic photoreceptors which overcome both such image problems.
Original image is frequently prepared in addition to the copy image preparation, and higher quality of images are required in the digital type electrophotography.
An image forming method employing a polymerization toner is proposed for an example of technique to obtain higher image as disclosed in Japanese Patent Open to Public Inspection No. 2000-214629. The polymerization toner has spherical shape and it has greater adhesive force to a photoreceptor, and therefore, induces a problems such as reducing transferring characteristics to a image forming sheet from the photoreceptor or cleaning characteristics, whereby the sufficient image density is not expected and sometimes generates an image deficiency such as white spot mentioned above.
In order to simultaneously minimize the aforesaid black spots and white undeveloped spots, the inventors of the present invention noticed that conventional investigations, which had mainly been directed to the interlayer, were insufficient and conducted comprehensive investigations while including the charge generating layer through the charge transport layer other than the interlayer.